In recent years, carbon dioxide emitted from an internal combustion engine of an automobile and others has been a problem from the standpoint of protection of the global environment. A lean burn engine using less fuel provides a promising solution to this problem.
A catalyst including a NOx storage material, such as alkali metal and others, as well as a noble metal is used as a catalyst for purifying NOx contained in exhaust gas (see Patent Literature 1). This catalyst absorbs NOx contained in the exhaust gas into the NOx storage material. When fuel concentration in the exhaust gas becomes rich, the absorbed NOx is removed by the action of the noble metal.
Patent Literature 1: Unexamined Japanese Patent Publication No. 2002-361094